1. Field of the Invention
The invention relates to an abnormality determination apparatus for an oxygen sensor which determines the presence/absence of an abnormality of an oxygen sensor that is provided downstream of a catalytic converter of an internal combustion engine, and also to an abnormality determination method for the oxygen sensor.
2. Description of the Related Art
In internal combustion engines, removal of exhaust gas components has been carried out through the use of catalysts for use for exhaust gas purification which are provided in an exhaust passageway. The removal of exhaust gas components by catalysts is efficiently performed if the air/fuel ratio of the mixture burned in the internal combustion engine is within a predetermined range. Therefore, it is a common practice to provide an oxygen sensor that detects the oxygen concentration in exhaust gas at an upstream side of a catalyst, and perform an air/fuel ratio feedback control of detecting the air/fuel ratio of mixture on the basis of an output signal of the oxygen sensor, and finding an air/fuel ratio correction value for the amount of fuel injection such that the detected air/fuel ratio becomes equal to a target air/fuel ratio, and correcting the amount of fuel injection to the increased or decreased side through the use of the air/fuel ratio correction value.
Furthermore, in order to grasp the state of removal of exhaust gas components by a catalyst, it is also a common practice to provide an oxygen sensor at the downstream side of the catalyst, and perform a so-called subsidiary feedback control of air/fuel ratio in which the air/fuel ratio of the exhaust gas having passed through the catalyst is detected on the basis of the output value of the oxygen sensor, and an amendment value for the air/fuel ratio correction value is calculated according to the detected air/fuel ratio.
In the foregoing subsidiary feedback control, since the output value of the oxygen sensor at the downstream side of the catalyst is utilized, occurrence of abnormality in the oxygen sensor will give rise to a risk of failing to perform normal control and, in turn, failing to sufficiently purify exhaust gas.
Therefore, it has been proposed to adopt an apparatus that executes a fuel cut of stopping the fuel injection from fuel injection valves, and then determines the presence/absence of an abnormality of the oxygen sensor on the basis of the degree of change in the output value of the oxygen sensor at the downstream side of the catalyst after the output value begins to change to a value that shows a leaner state at the time of introduction of fresh air into the exhaust passageway (e.g., see Japanese Patent Application Publication No. 9-170966 (JP-A-9-170966), Japanese Patent Application Publication No. 2003-343339 (JP-A-2003-343339), and Japanese Patent Application Publication No. 2008-169776 (JP-A-2008-169776)).
For example, the apparatuses disclosed in JP-A-9-170966 and JP-A-2008-169776 measure as a response time the amount of time that is taken for the output value of the oxygen sensor at the downstream side of the catalyst to change from a first threshold value to a second threshold value that shows a leaner state than the first threshold value after the fuel cut is executed. Then, if this response time is longer than a reference response time that is set beforehand, it is determined that the oxygen sensor at the downstream side of the catalyst is abnormal.
By the way, even when the oxygen sensor at the downstream side of the catalyst is not abnormal, it sometimes happens that the output value of the oxygen sensor gradually changes to a value that shows the leaner state, during the period after the fuel cut is executed but before fresh air reaches the oxygen sensor at the downstream side of the catalyst (hereinafter, this phenomenon will be referred to also as “pre-fresh-air-arrival output decline phenomenon”). If despite occurrence of this phenomenon, the abnormality determination regarding the oxygen sensor is performed on the basis of the response time, there is possibility of making a false determination that the oxygen sensor is abnormal when the oxygen sensor is actually normal.
In order to reduce such false determinations as much as possible, the apparatus disclosed in JP-A-2008-169776 determines whether or not fresh air has reached the oxygen sensor at the upstream side of the catalyst, and detects the output value produced by the catalyst-downstream-side oxygen sensor immediately after it is determined that fresh air has reached the upstream-side oxygen sensor. Then, if the detected output value is less than or equal to a criterion value that is set beforehand, the apparatus prohibits the determination that the catalyst-downstream-side oxygen sensor is abnormal.
As stated above, the apparatus disclosed in JP-A-2008-169776 determines whether or not the foregoing pre-fresh-air-arrival output decline phenomenon has occurred on the basis of the output value of the catalyst-downstream-side oxygen sensor that is produced immediately after the arrival of fresh air at the catalyst-upstream-side oxygen sensor. However the accuracy of this determination cannot be said to be sufficiently high, and therefore has been desired to be further improved.